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Open Thesis FINAL Sporer.Pdf THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE DEPARTMENT OF MECHANICAL AND NUCLEAR ENGINEERING A METHOD FOR ACTIVE SPACE CHARGE NEUTRALIZATION IN AN INERTIAL ELECTROSTATIC CONFINEMENT (IEC) NUCLEAR FUSION DEVICE BRENDAN SPORER SPRING 2017 A thesis submitted in partial fulfillment of the requirements for baccalaureate degrees in Nuclear Engineering and Mechanical Engineering with honors in Nuclear Engineering Reviewed and approved* by the following: Sven G. Bilén Professor of Engineering Design, Electrical Engineering, and Aerospace Engineering Thesis Supervisor Seungjin Kim Professor of Nuclear Engineering Honors Adviser * Signatures are on file in the Schreyer Honors College. i ABSTRACT Recent inertial electrostatic confinement (IEC) fusion concepts are discussed and their shortcomings noted. Ion space charge is substantiated as a significant hindrance to high efficiencies, so a method for space charge neutralization in an ion-injected IEC device is proposed. An electrostatically- plugged magnetic trap is used to confine electrons in the core region of a planar electrostatic trap for ions. The electrons act to dynamically neutralize the space charge created by converging ions for the purpose of increasing achievable core density and fusion rates. An electrostatic trap utilizing this method of neutralization is termed the plasma-core planar electrostatic trap, or PCPET. COMSOL Multiphysics 4.3 is used to model the electromagnetic fields of the PCPET and compute lone ion and electron trajectories within them. In the proper configuration, ions are shown to be stably confined in the trap for many hundreds of oscillations, potentially much longer. Electrons are confined virtually infinitely in the central electrostatically-plugged cusp. For both species, upscatter into source electrodes seems to be the dominant loss mechanism. Adjusting the electron energy and behavior in the core to provide the optimum neutralization for ions is discussed. Ion synchronization behavior can be controlled with RF signals applied to the anode. Two operational modes are identified and discriminated by the state of ion synchronization. Further experimentation is needed to determine which mode produces the optimal neutralization and fusion rate. An experimental prototype PCPET is constructed out of 3D-printed PLA and machined aluminum. ii TABLE OF CONTENTS LIST OF FIGURES ..............................................................................................................iii ACKNOWLEDGMENTS .....................................................................................................v Chapter 1 Introduction to Inertial Electrostatic Confinement (IEC) Fusion ..........................1 Overview of Thesis .................................................................................................................... 3 Chapter 2 Problems with Traditional IEC Experiments ........................................................4 Ion Grid Impact .......................................................................................................................... 4 Operation Pressure ..................................................................................................................... 5 Beam–Background vs. Beam–Beam Reactions ......................................................................... 5 Space Charge Limitations .......................................................................................................... 6 Streaming Electrons ................................................................................................................... 7 Chapter 3 Second-Generation IEC Experiments ...................................................................9 McGuire’s Multi-Grid IEC [McGuire, 2007] ............................................................................ 9 Synchronized “Bunching” Phenomena ...................................................................................... 11 Periodically Oscillating Plasma Sphere (POPS) [Park, 2005] ................................................... 14 Klein’s MARBLE Concept [Klein, 2011].................................................................................. 16 Planar Electrostatic Trap (PET) [Knapp, 2015] ......................................................................... 19 Conclusions to Be Drawn........................................................................................................... 21 Chapter 4 A Plasma-Core Planar Electrostatic Trap ..............................................................24 The Saddle Point Potential ......................................................................................................... 25 Magnetic Electrostatically-Plugged Confinement [Ware, 1969] [Dolan, 1994] ........................ 26 The Plasma-Core Planar Electrostatic Trap ............................................................................... 29 COMSOL Multiphysics Simulations with Ions ......................................................................... 30 COMSOL Multiphysics Simulations with Electrons ................................................................. 41 Central Plasma Design ............................................................................................................... 43 Ion Effects on Central Plasma .................................................................................................... 45 Possible Use of Other Fusion Fuels ........................................................................................... 46 Chapter 5 PCPET Prototype ..................................................................................................48 Chapter 6 Conclusions & Implications ..................................................................................51 BIBLIOGRAPHY..................................................................................................................54 iii LIST OF FIGURES Figure 1: A traditional "fusor" IEC device [FuseNet, 2017] .................................................1 Figure 2: Electric potential map of a traditional ion-injected fusor [McGuire, 2007] ...............................................................................................................................2 Figure 3: Double potential well in a traditional IEC device cathode [Miley, 2014] .............7 Figure 4: Multi-grid 2-D electric potential map [McGuire, 2007] ........................................10 Figure 5: Long-lived, stable synchronization at over 2,500 passes (0.01s) [McGuire, 2007] .............................................................................................................12 Figure 6: Steady-state ion map at 1mA (left) and 100mA (right) injection current [McGuire, 2007] .............................................................................................................13 Figure 7: Potential distribution along MARBLE axis [Klein, 2011] .....................................16 Figure 8: An electron Penning trap [PPPL, 2000] .................................................................17 Figure 9: MARBLE-1 device with ceramic spacers between electrodes [Klein, 2011] ...............................................................................................................................18 Figure 10: MARBLE-1 cross-section [Sedwick, 2013] .........................................................18 Figure 11: Axial potential distribution in MARBLE-1 [Klein, 2011] ...................................19 Figure 12: Planar electrostatic ion trap (halved) [Knapp, 2015] ...........................................20 Figure 13: Electric potential map of cross-sectional plane in Figure 12 [Knapp, 2015] ...............................................................................................................................20 Figure 14: 2-D saddle point electric potential map ................................................................26 Figure 15: A magnetic "bottle" plasma trap [Wesley, 2004] .................................................27 Figure 16: An electrostatically-plugged biconic (spindle) magnetic trap [Dolan, 1994] ...............................................................................................................................28 Figure 17: A plasma-core planar electrostatic trap (PCPET) ................................................30 Figure 18: Stable deuteron trajectory in the multi-grid at 100microseconds (>350 passes) .............................................................................................................................31 iv Figure 19: Stable deuteron (blue dot) trajectory in the PCPET with no central plasma at 100microseconds (>450 passes) .....................................................................33 Figure 20: Unusually stable deuteron (blue dot) trajectory in the PCPET with uniform electron plasma (n~5x1014) at 100microseconds (>450 passes) .......................34 Figure 21: Ten deuterons at 10microseconds (50 passes) in PCPET without plasma (top) and with uniform electron plasma (bottom) ..............................................35 Figure 22: Unstable deuteron trajectory in PCPET with Penning trap ion source ................36 Figure 23: A magnetic bottle (left) and a biconic/spindle cusp (right) – both capable of trapping charged particles with the magnetic mirror effect [Fusion Adv., 2015] .....................................................................................................................36
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